1. Technical Field of the Invention
The invention relates to an electron-optical corrector for eliminating both the third-order spherical aberration and the anisotropic (azimuthal) component of the third-order extra-axial coma using circular lenses and hexapole fields.
2. Description of the Prior Art
The efficiency of high-resolution imaging electron-optical systems, such as, for example, high-resolution electron microscopy, is limited by third-order spherical aberrations. Therefore, it is a principal concern in the modification of such systems to eliminate third-order spherical aberration. Another important criterion in the efficiency, besides high resolution, is to be able to image an image region of sufficient size. Its limit is determined in high-resolution electron-optical systems by the aberration of extra-axial coma, which is composed of the components of the radial coma, often also described as isotropic coma, and azimuthal coma, which is also known as anisotropic coma. Accordingly, the terms “radial” and “azimuthal” are used in one terminological system, and “isotropic” and “anisotropic” in the other terminological system. According to the conventional terminology, electron-optical imaging systems that contain no third-order spherical aberration and no extra-axial coma are termed aplanats. Derived from this, systems are termed semi-aplanats if, besides absence of third-order spherical aberration, they are only free of radial (isotropic) coma. In the case of electron-optical systems with a straight axis and circular Gaussian ray path, which are always assumed to be the case below, the third-order aberration consists of the third-order spherical aberration. The term spherical aberration covers all those optical image defects whose aberration integrals include only those Gaussian elementary paths that emerge from the optical axis in the object plane to be imaged. The magnitude of the object/image field to be imaged is determined by extra-axial aberration, that is to say, those aberrations whose aberration integrals also contain elementary paths according to Gaussian dioptrics that emerge in the object plane outside the optical axis. In high-resolution electron optics, the image aberration of extra-axial coma is the main factor limiting the image field.
German patent application DE 198 02 409, for which Haider et al., U.S. Pat. No. 6,605,810 B1, issued Aug. 12, 2003, is an English-language equivalent, describes a corrective in which two hexapoles are used, between which a round-lens doublet is interposed. Between the corrective comprising the lenses and the objective lens, a further circular lens is interposed such that the coma-free plane of the objective lens is imaged in the coma-free plane of the corrective. But the term “coma-free plane,” or more precisely “coma-free diaphragm plane,” strictly speaking, describes the property “free of isotropic (radial) coma.” A corrective of this construction is thus the elimination of the third-order spherical aberration of the objective lens, avoiding the radial (isotropic) component of the extra-axial coma. According to the conventional terminology detailed above, the system, comprising objective lens, transfer system and corrective, in result, represents a semi-aplanat.
In the above-described arrangement, it is a disadvantage that, as a result of the unavoidable anisotropic extra-axial coma of the magnetic objective lens, which is not influenced by the corrective, a restriction of the focused image region takes place, which could be considerably enlarged by complete elimination of this image aberration.